This project examines mitochondrial functioning in old age and in pathological states in which decreased energy transduction by mitochondria may compromise tissue survival. (1) We have focused this year on the relation between altered mitochondrial Ca2+ ion homeostasis and the potential for oxidative phosphorylation. We have extended our previous work on cardiac myocytes from the cardiomyopathic Syrian hamster. Studied at the point of failure, these hearts perform less work and also fail to activate pyruvate dehydrogenase as completely as in healthy hearts. In the previous year we tied this to a failure to elevate intramitochondrial free Ca2+ ([Ca2+m) in cells from myopathic animals, when [Ca2+]m was studied in individual cardiac myocytes subjected to an electrical pacing regimen. This year we showed that the likely cause of the diminished response of [Ca2+]m to increased frequency of electrical stimulation in the cardiomyopathic is the generation of smaller systolic transients in cytosolic free Ca2+ ([Ca2+]c) in myocytes from the failing hearts. (2) We have extended the previous year's studies on the dependence of [Ca2+]m upon frequency of electrical stimulation, to separate an effect due to beta-adrenergic stimulation from that of frequency. Further, we have used the effect of mitochondrial uncoupling agents to provide an independent verification of our finding that the mitochondrial Ca2+ gradient may be positive or negative, depending upon the degree of cell stimulation.